184 Notes on the Area of Intrusive Rochs at Bar go. 



example was laid open in a shaft which had been sunk upon 

 a small auriferous quartz vein at the Perseverance mine in 

 Orr's Gully. The contact plane at this place is nearly 

 horizontal, and crosses the shaft about 20 feet from the 

 surface. The beds nearest to the intrusive rock have assumed 

 an extreme form, being almost crystalline granular, and 

 distantly resembling the igneous rocks in appearance. A 

 sample from this place is spotted in places with small oval 

 patches of a yellowish tint, and which, when examined by 

 the lens, have a crystalline-granular structure and a "granitic" 

 appearance, being formed of felspar, mica, and quartz, 

 showing the influence of the adjoining intrusive rocks. 



Examined in a thin slice, it proves to be crystalline- 

 granular, and to be composed of — (a) Very angular or even 

 cavernous felspars, most of which are orthoclase, the triclinic 

 felspars being both few in number and small in size ; (6) 

 numerous angular grains and clusters of grains of quartz ; 

 (c) ragged flakes and crystals of brown mica, which is in 

 parts much bleached in colour, scarcely pleochroic, and having 

 the iron eliminated as crystals of secondary magnetite. 

 This mica is one of the first formed of the minerals, as it is 

 included both in the felspars and in the quartz, {d) Very 

 fibrous masses of yellow or colourless alkali mica or aggre- 

 gations of flakes, or plumose or stellate groups of the same, 

 with a few larger isolated plates. This mica seems to be a 

 late-formed mineral, and may in part be a secondary product 

 of alteration. There is also some magnetite, which may be 

 original. 



This sample shows the peculiar metamorphic action upon 

 sediments immediately adjoining the contact, not only in the 

 complete recrystallisation of the materials of the sediment, 

 but also in the generation of felspars, which are not to be 

 found in the hornfels rocks at a distance. 



The changes which can be traced out in proceeding from 

 the normal sediments at the outside of the metamorphic 

 zone towards the intrusive, rock masses are mainly the 

 conversion of the argillaceous material of the former into 

 mica of two kinds, in the general silicification of the altered 

 rocks, and finally, near to the contact, the complete molecular 

 recrystallisation of the sediments with the production of 

 felspars in those beds which are at touch with the invasive 

 rocks. 



It may be inferred that not only has there been an elimi- 

 nation of free silica during the alteration of the sediments. 



